The present invention relates generally to semiconductor detectors for detecting electromagnetic radiation and configured to provide internal gain from detected radiation. More specifically, the present invention relates to an avalanche photodiode and method of manufacture therefor.
For several decades photomultiplier tubes (PMTs) have been prevalently used for sensing light for medical imaging, particle physics research, and the like. High energy particle physics experiments often include the use of hundreds and even thousands of PMTs to detect particles of various masses, various energies, and various angles of incidence. Different sized PMTs and different sensitivity PMTs are often used in particle detection experiments to provide for the detection of photons and particles having various properties. One example of a detector that includes a number of PMTs is the Super Kamiokande detector in Japan. The Super Kamiokande detector includes an approximately 50,000 ton tank of pure water viewed by over 11,000 photomultiplier tubes that are each approximately 50 centimeters in diameter.
One advantage of PMTs in particle detection applications is their relatively large amplification, which is often about 106. PMTs also provide the advantage of generating relatively low noise signals, which enables these devices to achieve high sensitivity for single particle detection. Although PMTs have been relatively prevalently used for particle detection applications, PMTs also have a number of inherent limitations. Specifically, PMTs are relatively costly, and large PMTs generally are not easy to mass produce. Further, PMTs are generally not operable under pressures exceeding a few atmospheres as the glass from which PMTs are fabricated tends to implode. Further yet, PMT sensitivity is often limited to a relatively small wavelength band, and the optical quantum efficiency of PMTs is also relatively low. Further still, PMTs generally are not operable under relatively high magnetic fields as these magnetic fields tend to push avalanche electrons from their intended paths within the PMTs. And, PMTs tend to be bulky. As a result, in many medical imaging applications, high energy applications, and nuclear physics applications, there remains a need for alternative designs of large photosensors that address the limitations of PMTs.
One solid state substitute for PMTs is the avalanche photodiode (APD). An APD is a semiconductor device that may be reverse biased near the breakdown region of the device such that multiple charges are generated in this region as a result of the absorption of an incident photon. The generated charge is amplified in the APD as a result of a cascading effect as the charge is accelerated by the reverse bias potential applied across the p-n junction of the device. An APD (similar to a PMT) exhibits internal gain created by an impact ionization process in the device, but an APD (disparate from a PMT) exhibits relatively high optical quantum efficiency. For example, the optical quantum efficiency of an APD may be four times (or more) higher than the optical quantum efficiency of a PMT. Further, APDs tend to exhibit a wider spectral response as compared to PMTs. Further yet, APDs tend to be insensitive to externally applied magnetic fields, unlike PMTs.
Early APDs were often discrete devices having beveled edges to prevent premature voltage breakdown at the sides of these APDs to thereby achieve “high” gain. See, for example, the beveled edge APDs described in U.S. Pat. Nos. 3,293,435, 3,491,272, and 3,449,177 of Huth. Specifically, beveled edges are used to reduce the peak surface electric field (i.e., the electric field across the p-n junction in the area where the p-n junction meets the surface of the APD structure) substantially below the peak bulk electric field (i.e., the electric field across the p-n junction in the body of the device where the p-n junction is disposed substantially parallel to the surfaces of the device to which the bias voltage is applied). Reducing the peak surface electric field generally inhibits breakdown at the surfaces and provides, instead, for primary breakdown in the bulk. Breakdown at the surfaces tends not to contribute measurable signal current and tends to contribute leakage currents as well as other undesired effects, such as device noise. For example, the peak surface electric field may have a value about 70% or less than the value of the peak bulk electric field to provide that the APD breaks down in the bulk.
Beveling is typically achieved via manual fabrication processes, such as grinding, and as a result, the cost of fabricating beveled edge APDs is generally high (˜$1000 for a 1-2 square centimeter (cm2) device). Relatively large APDs that are typically commercially available and that have this design are typically about 2 cm2 or less. Due to the high cost and relatively small size of beveled edge APDs, these APDs tend not to be suitable for many medical imaging applications and high energy physics applications.
Methods have been investigated to avoid the beveled configuration of APDs. U.S. Pat. Nos. having numbers 5,670,383 and 5,446,408 of Piccone. However, the described APDs in these patents tend to have a relatively thin portion, which renders these APDs relatively fragile. The disclosures of U.S. Pat. Nos. 5,670,383 and 5,446,408 are incorporated by reference herein in their entireties.
Therefore, the need exists for new APDs and new APD manufacturing techniques, wherein the APDs are relatively robust and easy to fabricate at relatively low cost.